The use of polyurethane resins in coating compositions is well known. These resins are generally synthesized by treatment of diisocyanates with diols. Such resins are disclosed in U.S. Pat. No. 4,794,147. Polyurethane resins are widely used for both basecoat and clearcoat coating compositions. There is however, ongoing research to develop resins and coatings with improved performance characteristics. One area of performance concerns the resistance of a cured coating to stone chipping and impact resistance.
The damage caused by chipping and impact is particularly problematic in the area of vehicle coatings. The availability of a resin and coating that provides improved chip and impact resistance provides an important advance in the art. Costly repairs to coatings damaged from stone chipping and other impacts could be greatly reduced, and the life of a coated article prolonged.
It has unexpectedly been found that polyamide modified polyurethane resins are useful in a coating composition, because the polyamides themselves are highly viscous and have low solubility. It is also surprising, because of the low solubility of the polyamides, that the polyamide modified polyurethane resin has excellent water dispersibility properties. The coating containing the polyamide modified resin exhibits a high degree of transparency and provides a high quality clearcoat. Additionally, the coatings demonstrate good resistance to high humidity conditions.
This invention, therefore, relates to the field of polyurethane coatings for use in automobile basecoat/clearcoat systems. In particular, this invention relates to the discovery that polyamide modified polyurethane resins and coating compositions prepared therefrom demonstrate a high degree of resistance to stone chipping and damage caused by both direct and reverse impact. These coating compositions exhibit coating characteristics superior to those taught by the art and are especially useful as clearcoats and basecoats for automobiles.